Silicones (polyorganosiloxanes) having acrylic, epoxy, photoinitating or other organic functional groups bound to silicon atoms by various linkages are well known. Recently, there have been described silicones with cluster acrylic functionality, that is multiple acrylic groups spatially concentrated near each other. Thus, in U.S. Pat. No. 4,387,240 there are described low molecular weight oligomers having a high methacrylate density which may be represented by the formula: ##STR3## Where m is 1, 2 or 3; p is 2-10 and Q is methacryloxypropyl or similar groups. These oligomers are used as binders in dental filling compositions to give hard abrasion resistant cured fillings. Also, in U.S. Pat. No. 4,424,328 there are described branched low molecular weight siloxane compounds with 3 or 4 of the branches terminated with methacryloxypropyl groups. These compounds are used in the preparation of hard contact lenses.
In copending applications, U.S. Ser. Nos. 623,759, now U.S. Pat. No. 4,575,546, 623,760, now U.S. Pat. No. 4,575,545 and 723,791, filed June 22, 1984, now abandoned, it is disclosed that good elastomeric properties and much more rapid cure times are obtained by block silicones of the formula (AB).sub.n C where A is a relatively short polymer segment containing at least 3 siloxane repeat units having acrylic functionality, B is a longer segment containing no acrylic functionality and C is A or a triorganosilyl group such as methacryloxypropyldimethylsilyl. See also U.S. Pat. No. 4,504,629.
In Au20051/83 there are disclosed reaction products of silicone carbinols, diisocyanates and hydroxy functional ethylenically unsaturated compounds such as hdyroxyethyl methacrylate. This reference suggests, does not exemplify the use of triisocyanates and/or polyacrylic hydroxyl compounds such as pentaerithitol di- or tri-acrylate. Such products are disclosed as useful for contact lenses where high oxygen permeability is desired. It is well known, however, that urethane linkages have poor high temperature resistance and the oxygen permeability of the silicone portion of the molecule would be expected to further errode the high temperature stability of the urethane linkage.
In U.S. Pat. No. 4,293,397 there are disclosed silicones of the general formula: ##STR4## where R.sup.1 is a divalent hydrocarbon group, R.sup.2 is a monovalent hydrocarbon group, X is a hydroxy group or a hydrolyzable group, k is 0 or a positive integer not exceeding 4, m is a positive integer from 50 to 10,000, a is 0, 1 or 2 and at least two of the groups denoted by A in the molecule are each a acryloxy-containing group represented by the formula. ##STR5## R.sup.3 being a hydrogen atom or an alkyl group having from 1 to 4 carbon atoms and n being a integer from 1 to 10, the remainder of A being each a hydrogen atom.